To date, the semiconductor material most commonly used in semiconductor-on-insulator structures has been single crystalline silicon. Such structures have been referred to in the literature as silicon-on-insulator structures and the abbreviation “SOI” has been applied to such structures. Silicon-on-insulator technology is becoming increasingly important for high performance thin film transistors, solar cells, and displays. Silicon-on-insulator wafers consist of a thin layer of substantially single crystal silicon 0.01-1 microns in thickness on an insulating material. As used herein, SOI shall be construed more broadly to include thin layer material and insulating semiconductor materials other than and including silicon.
Various ways of obtaining SOI structures include epitaxial growth of silicon on lattice matched substrates. An alternative process includes the bonding of a single crystal silicon wafer to another silicon wafer on which an oxide layer of SiO2 has been grown, followed by polishing or etching of the top wafer down to, for example, a 0.05 to 0.3 micron layer of single crystal silicon. Further methods include ion-implantation methods in which hydrogen ions are implanted into a donor silicon wafer to create a weakened layer in the wafer for separation (exfoliation) of a thin silicon layer that is bonded to another silicon wafer with an insulating (or barrier) oxide layer in between. The latter method involving hydrogen ion implantation is currently considered advantageous over the former methods.
U.S. Pat. No. 5,374,564 discloses a “Smart Cut” hydrogen ion implantation thin film transfer and thermal bonding process for producing SOI substrates. Thin film exfoliation and transfer by the hydrogen ion implantation method typically consists of the following steps. A thermal oxide film is grown on a single crystal silicon wafer (the donor wafer). The thermal oxide film becomes a buried insulator or barrier layer between the insulator/support wafer and the single crystal film layer in the resulting of SOI structure. Hydrogen ions are then implanted into the donor wafer to generate subsurface flaws. Helium ions may also be co-implanted with the Hydrogen ions. The implantation energy determines the depth at which the flaws are generated and the dosage determines flaw density at this depth. The donor wafer is then placed into contact with another silicon support wafer (the insulating support, receiver or handle substrate or wafer) at room temperature to form a tentative bond between the donor wafer and the support wafer. The wafers are then heat-treated to about 600° C. to cause growth of the subsurface flaws resulting in separation of a thin layer or film of silicon from the donor wafer. The assembly is then heated to a temperature above 1000° C. to fully bond the silicon to the support wafer. This process forms an SOI structure with a thin film of silicon bonded to a silicon support wafer with an oxide insulator or barrier layer in between the film of silicon and the support wafer.
As described in U.S. Pat. No. 7,176,528, the ion implantation thin film separation technique has been applied more recently to SOI structures wherein the support substrate is a glass or glass-ceramic sheet rather than another silicon wafer. This kind of structure is further referred to as silicon-on-glass (SiOG), although semiconductor materials other than silicon may be employed to form a semiconductor-on-glass (SOG) structure. One potential issue with SiOG is that the glass support or handle substrate contains metal and other constituents that may be harmful to the silicon or other semiconductor layer. Therefore, a barrier layer may be required between the glass substrate and the silicon layer in the SiOG. In some cases, this barrier layer facilitates the bonding of the silicon layer to the glass support substrate by making the bonding surface of the silicon layer hydrophilic. In this regard, an SiO2 layer may be used to obtain hydrophilic surface conditions between the glass support substrate and the silicon layer. A native SiO2 layer may be formed on the donor silicon wafer when it is exposed to the atmosphere prior to bonding. Additionally, the anodic bonding process produces “in situ” SiO2 layer between the silicon donor wafer or exfoliation layer and the glass substrate. If desired, an SiO2 layer may be actively deposited or grown on the donor wafer prior to bonding. Another type of a barrier layer provided by the anodic bonding process disclosed in U.S. Pat. No. 7,176,528 is a modified layer of glass in the glass substrate adjacent to the silicon layer with a reduced level of ions. Anodic bonding substantially removes alkali and alkali earth glass constituents and other positive modifier ions that are harmful for silicon from about 100 nm thick region in the surface of glass adjoining the bond interface.
The anodically created substantially alkali free glass barrier layer and the in situ or deposited SiO2 barrier layers may, however, be insufficient for preventing sodium from moving from the glass substrate into the silicon layer. Sodium readily diffuses and drifts in SiO2 and glasses under the influence of an electric field at slightly elevated temperatures, even at room temperature, possibly resulting in sodium contamination of the silicon layer on the glass substrate. Sodium contamination of the silicon layer may cause the threshold voltages of transistors formed in or on the SiOG substrate to drift, which in turn may cause circuits built on the SiOG substrate to malfunction.
Another potential problem observed with SiOG substrates produced with an ion implantation film transfer process is the occurrence of micro structural defects in the thin silicon layer transferred during exfoliation of the silicon layer (the exfoliation layer) from the donor wafer. FIGS. 1 through 3 show a typical surface morphology of an as transferred silicon exfoliation layer on a glass substrate. FIG. 1 is an atomic force microscope (AFM) image of the surface of a typical as transferred silicon exfoliation layer and FIG. 2 is a plot of the surface topography of this as transferred surface along line 3 in FIG. 1. By as transferred, it is meant that the exfoliation layer has not undergone any surface finishing or processing following exfoliation of the layer from the donor wafer. As can be observed in FIGS. 1 and 2, characteristic features of the as transferred silicon exfoliation layer (or simply as transferred layer) include relatively flat mesas 10 surrounded by canyons 20 that include craters or pin holes 30 that extend deeply into the silicon layer 122. The pin holes 30 may penetrate entirely through the silicon layer 122 to the underlying glass substrate 102. When transistors are made in the silicon layer with the canyons 20 and pin holes 30, the canyons and pin holes are likely to disrupt proper transistor formation and operation. The canyons and pin holes are believed to be a consequence of the ion implantation silicon film transfer process when bonding and transferring the silicon film to a glass support substrate.
There is a need in the art for a process and structure for preventing or at least mitigating the occurrence of canyons and pinholes in the semiconductor layer in SiOG or SOG products fabricated using ion implantation thin film transfer processes.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinence of any cited documents.